This invention relates to pneumatic conveying devices in general and in particular to a line charging or pumping device for a pneumatic conveying system wherein solid particulate material to be conveyed is supplied from a source to a conduit where it is conveyed by gaseous fluid under pressure to a remote location. More particularly, the present invention relates to a pneumatic conveying device of the type which includes a screw impeller rotatably mounted in a casing for transporting materials from a source such as a hopper through the casing to a discharge chamber. Gaseous fluid under pressure is supplied to the discharge chamber for entraining material and conveying it from the discharge chamber through a conveying line. The device is particularly suited to conveying large quantities of abrasive materials such as cement raw meal or finished cement and pulverized coal over long distances.
Early designs of pneumatic conveying devices of the type to which the present invention relates were generally shown in U.S. Pat. No. 1,677,119. This design utilizes front and rear bearings to rotatably support a screw impeller within the bore of a casing. A compressing screw formed by either a reducing pitch screw or a reducing diameter casing bore or an increasing diameter screw impeller shaft or combination thereof serves to compact material within the bore to thereby form a seal of material in the casing between the material outlet and the material inlet of the casing. This seal of material serves to prevent "blowback" from the discharge chamber into the feed hopper of the device. This "blowback" takes the form of conveying gas under pressure short circuiting through the casing of the apparatus into the material hopper rather than serving to convey the material out of the discharge chamber and through the pneumatic conveying line.
A later design or a pneumatic conveying device of the type to which the present invention relates eliminated the front bearing. With this design, the screw impeller was supported only by rear bearings. The front of the screw is supported in part by material surrounding the screw. Material is discharged from the bore of the casing through an open end by the screw impeller into a discharge chamber. While a compressing screw of the type previously described was still utilized in this design, in order to aid in the formation of a material seal, a flap valve is positioned at the end of the casing for normally closing the end of the casing. This flap valve is openable by means of the material being conveyed through the casing exerting a force on the flap valve. During normal operation, the "blowback" previously described is prevented by the seal of material, but during times when conveying at less than capacity or when material supply through the casing to the discharge chamber is stopped, the flap valve closes to prevent blowback. This design is illustrated by U.S. Pat. No. 2,299,470.
A further development in pneumatic conveying devices of this type is shown in U.S. Pat. No. 4,109,966 and British Patent Specification No. 1,435,444. In that design, the use of both rear and front bearings as illustrated in U.S. Pat. No. 1,677,119 is combined with the use of a normally closed flap valve for aiding in the formation of a material seal as illustrated in U.S. Pat. No. 2,299,470.
It has been generally accepted by the prior art that it is necessary to include nozzles in the discharge chamber or windbox at or near the location of the outlet from the casing and inlet to the discharge chamber for the purpose of directing high velocity conveying gas into the discharge chamber. It was believed that localized high velocity gas was necessary to distribute the air flow over the entire discharge chamber and quickly fluidize the material being supplied to the discharge chamber. The air then expanded through the discharge chamber and served to entrain the material through the outlet of the discharge chamber and convey it through the conveying line.
Also with the prior art, it was believed in both the arrangement where the screw was supported at both ends such as in U.S. Pat. No. 1,677,119 and where the screw was supported at one end such as in U.S. Pat. No. 2,299,470 that only a single outlet from the casing was necessary or appropriate. With the present invention, it has been determined that more than one outlet from the casing can be utilized while supporting the screw impeller at both ends. The use of multiple outlets is believed to result in improved conveying capacities at reduced power consumption.
With the apparatus of the prior art, it is usually necessary to orient the pump or conveying apparatus in a specific position relative to the direction in which material is to be conveyed. With the early devices, it is necessary to orient the pump so that it is in line with the initial conveying direction, i.e. the axis of the pump screw must be parallel with the axis of the outlet of the conveying device and inlet of the conveying line. With the device shown in U.S. Pat. No. 4,109,966, the material pump or line charger has to be oriented in a direction perpendicular to the outlet of the discharge chamber and inlet of the conveying line. With the present device, the conveying line can be connected to either side or parallel to the pumping device and the inlet for compressed gas can be placed on the other side. Alternatively, the gas inlet and material outlet can be arranged parallel to the pumping device. This arrangement improves flexibility of system layout.
The use of multiple outlets from the casing of the apparatus and the elimination of nozzles between the conveying source and the conveying line results in improved capacity and a reduction in power requirements for the apparatus. The multiple outlets are believed to reduce power consumption required to rotate the screw impeller. The elimination of the nozzles in the discharge chamber serves to reduce pressure drop and thus reduce energy consumption of the means compressing the conveying gas. It is also believed that the multiple outlets result in less wear of the various components of the device.